Projectile speed sensor

[Mike] built a sensor rig to measure projectile speed. The setup uses a tunnel with two sensors in it. Each consists of a laser diode on one side focused on a photodiode in the other. The two are monitored by an op amp and measured by an ATmega128 microcontroller. When the beams are broken the elapsed time between the two events is measured in order to calculate speed. There is a setting to adjust the calibration for a range of speeds, which came in quite handy as [Mike] initially tested the device with rubber bands before moving on to a pellet gun and then a rifle.

i’d like to request that someone set up a shitload of cameras in metered sequence with a device like this. set them up in a circular arch above and all pointed at the center of the target. use a very rigid target so i can watch a bullet collapse from a pretty, moving angle.

Good project. If you are interested in using the data for shooting, I would suggest placing your rig like a chronograph, i.e., at a known, relatively short distance from the muzzle. The closer your sensors to the muzzle, the easier it will be to consistently have shots that will break the beam. Also, testing at a shorter distance from the muzzle will minimize the effects of external ballistics on your rounds (density, temperature, humidity, pressure, wind), which will make it easier to confirm your results with published data.

“That is, when they would fire – a number of them were just a bit too long to fit completely in the chamber. I’m not sure whether this was a defect or if they were just too big for this particular gun.”

When I was a kid we used to fire long-rifle .22 rounds with a rifle that wasn’t quite made for them, they’d fire but wouldn’t eject properly. All it took was a tool to grab the shell and pull it out, no big deal. :p

Using the input capture unit and letting the counter run continuously seems like an obvious thing to do in this situation. It’d improve accuracy by a couple of cycles; I don’t see him accounting for the interrupt delay in his code. Seems like it’d also be worthwhile to add the couple lines to the overflow handler to eliminate the need for the slow mode and increase the timeout.

I think he could have used the internal analog comparator as well, the ADC mux should be fast enough for this usage, though it’ll complicate the code a bit, it’d eliminate external components.

@Spork – yea, I only figured out that this was called a Chronograph yesterday afternoon. :)

@binary – thanks for the tips! Are you then less worried about muzzle flash affecting the electronics? And yes, the quality of those Magtech rounds was pretty pathetic.

@andar_b – sounds like we’ve got similar problems, although my 45 rounds won’t even fire – the slide won’t close and lock…

@error404 – thanks for the tips! believe it or not i’ve never tried input capture – i’ll have to put it on my list. and you’re right, there are a lot of things i could have done better here. i spent probably 30 minutes putting the whole thing together and then at least three times that on the write-up. guess i’ve got something backwards. ;)

What Spork said. It’s a chronograph, though perhaps one with a attractive price point. Homebrew ones up till now required one to shoot through two spinning disks of paper. I’ve had an idea to make a crude one with the input jack of a sound card, and Audacity, but I’ve yet to get a good look at how the “skyscreens” work. I think they are LEDs, reflective tape, and photo-diodes.

>…a number of them were just a bit too long to fit completely in the chamber

yikes, either clean the gun, or call the manufacture, unless they are reloads. It’s remotely possible that the P+ loads are bumping bullets our of the cases, having not been properly crimped.

andar_b>When I was a kid we used to fire long-rifle .22 rounds with a rifle that wasn’t quite made for them, they’d fire but wouldn’t eject properly. All it took was a tool to grab the shell and pull it out, no big deal. :p

.22 long rifle in a .22 magnum gun with a trimmed 22 mag case for a chamber adapter? Not generally recommended, but doable in a pinch. Single shot only please. And remember, it’s your eye

Cool project. I’ve been thinking about a project where you would use a chronograph to determine bullet velocity combined with a IR grid of some sort that could also track bullet position. i.e. a virtual target. Anyone ever seen anything similar to this?

@zippyg – I was working on such project to track automated actual bullet position on a target with the help of IR grid. But can u tell me the details of the sensor and microcontroller required for that. What’s about the project?

@andar_b – as anyone will say, discharging a cartridge in a weapon that wasn’t made for that much power is a pretty big risk. now yes, it is true that modern gun barrels will perform far above their specifications FOR A FEW ROUNDS, but you don’t want to make it a regular practice to, for example, fire a .22LR through a stock .22 all day long; there is increased risk with every new overpowered shot.

it’s the same reason you want to avoid using +P loads in a gun that wasn’t made for them, and the same reason you want to avoid firing .357 magnum shells through a .38spc because seriously you’re taking unreasonable risks every time you squeeze that trigger.

At 3400fps, it should be good to +/- 9fps or about 0.25% (based on a country frequency of 16Mhz and a sensor spacing of 2″, 0.167′). Its programming allows measurements up to 10000fps, but it could theoretically measure even greater speeds – accuracy would start to decrease though.